Control circuit



May 26, 1936. D. D. KNOWLES CONTROL CIRCUIT Filed July so, 1932 flffl /4KY fl/l/ llllll P/d/ D e a INVENTOR flan e; Q Maw/es.

O WITNESSES:

ATTORNEY Patented May 26, 1936 UNITED STATES PATENT OFFICE CONTROLCIRCUIT Pennsylvania Application July 30, 1932, Serial No. 626,849

10 Claims.

My invention relates in general to precipitating devices for smokestacks and the like, and more particularly to the electrical circuitsfor supplying'a uni-directional current of a relatively high voltage tothe precipitating device.

In control circuits for precipitating devices employing high-voltagerectifying tubes to supply a uni-directional current of relativelyhighvoltage, it is imperative, to protect the rectifying tubes, that theload current be immediately reduced to a predetermined safe value justas soon as an overload condition or a fiashover occurs in theprecipitating device.

Whenever the density of the smoke varies during the course of theoperation of.a precipitating device or should dust and other foreignparticles collect and build up in the precipitating device, flashoversmay occur in the precipitating de-- vice. These flashovers give rise tosurges in the control circuits and principally the rectifying tubes. Ifthe flashovers are not prevented, they will eventually ruin therectifying tubes in the course of a relatively short period ofoperation. When a shortcircuit condition or a flashover ocours in theprecipitating device, the rectifying tubes, if not protected, mustabsorb substantially all of the energy of the short-circuit. In otherwords, the rectifying tubes cannot supply the heavy current demandduring a short-circuit condition, because their emission is limited tothe value of the saturation current that corresponds to the temperatureof the filament. As a result, the potential between the plate and thefilament of the rectifying tubes increases very rapidly untilsubstantially the full potential of the supply is applied between theplate and the filament of the rectifying tubes. The plate is thenbombarded by electrons which have acquired a velocity equiv alent to thefull supply potential, and unless the short-circuit condition orflashover is immediately removed, the plate may melt, or, because of thehigh temperature of the plate, the filament may be destroyed by backbombardment of the electrons from the plate on the other half cycle orboth plate and filament may be destroyed.

Consequently, the very frequent fiashovers incident to the operation ofa precipitating device makes the question of short-circuit protection avery important consideration, because of its direct bearing on the lifeof the rectifying tubes.

Heretofore, mechanical means have been employed to trip or disconnectthe rectifying tubes from the supply source. As is obvious, suchmechanical devices are too slow in their operation, both from thestandpoint of adequately protecting the rectifying tubes and of insuringeflicient.

operation of the precipitating device. All of these mechanical means maybe manifestly classed as remedial measures because they operate toremove a short-circuit condition and not to prevent it.

Therefore, it is an object of my invention to provide preventivemeasures rather than remedial measures to make it substantiallyimpossible for flashovers to occur in a precipitating device under anyoperating condition.

Another object of my invention is the provision of a control circuit ofthe class indicated that shall be simple and reliable in operation andbe economically manufactured and installed.

A further object of my invention is to provide for changing alternatingcurrent to uni-directional current and delivering the uni-directionalcurrent to a precipitating device, and for maintaining theuni-directional current at a substantially predetermined selected value.

It is also an object of my invention to provide for regulating thecurrent flowing through a gridcontrolled rectifying tube in accordancewith a negative blocking bias that is responsive to the said currentpassed by the rectifying tube.

A further object of my invention is to provide for causing theregulating action of the grid-controlled rectifying tubes to be of anypredetermined rate or sensitivity.

Other objects and a fuller understanding of my invention may be had byreferring to the following description taken in connection with theaccompanying drawing, in which:

Figure 1 is a diagrammatic view of a control system embodying thefeatures of my invention, and

Fig. 2 is a curve representing the volt-ampere characteristics of atwo-electrode thermionic tube which I employ in the illustrated controlcircuit of Fig. 1.

With reference to the drawing, my invention in general comprises aprecipitating device ill, a transformer 49 having a primary winding 50and a plurality of secondary windings 5!, 52, 53 and 54, a pair ofgrid-controlled rectifying tubes 30 and 38 for changing thealternating-current to a uni-directional current, and a two-electrodethermionic tube 46, in combination with condensers 40 and 4| and withadjustable resistors 42, 43, 44, and 45, for regulating the currentflowing through the grid-controlled rectifying tubes 30 and 38.

The electrical precipitating device [0 comprises an enclosure II thatsurrounds the base portion of a smoke stack for steel mills and thelike. Mounted within the enclosure H and surrounding the smoke stack arepositioned a nest of vertically disposed pipes |2. Disposed axially andextending through each of the vertically disposed tubes I2 is aconductor l4. The vertically disposed tubes |2 are generally referred toas collecting tubes or electrodes, and the conductors M as dischargeelectrodes, and will hereinafter be referred to as such. For the purposeof maintaining the discharge electrodes |4 taut, weights l3 aresuspended from the lower ends thereof.

As illustrated, the smoke or gases enter the upper portion of theenclosure II from the right and thence are forced downwardly through thenest of vertical collecting electrodes I2 and then upwardly through thesmoke stack to the atmosphere. When the electrical precipitating deviceis in operation, the discharge electrodes M are charged with auni-directional current of extremely high negative potential relative tothe collecting electrodes |2,'which may be connected to the ground l5.In actual practice, the potential may vary from 30,000 to 75,000 volts,depending upon the design of the installation.

The removal of the particles suspended in the smoke or gas as it passesdownwardly through the vertically disposed collecting electrodes I2 isaccomplished by the utilization of a strong electrical field that isestablished around each of the discharge electrodes |4. Such a strongfield causes ionization of the gas and is accompanied by the familiarcorona discharge. When the gas is ionized, the negative electrons of themolecules are separated from the positively charged nuclei or positiveions. These electrons may remain free or may unite with other neutralmolecules to form negative ions.

The ions and electrons become attached to the suspended particles in theair and thereby cause the suspended particles to become negativelycharged. Hence, the negatively charged particles then behave inaccordance with the law of electrostatic traction, and, in consequence,are repelled by the negatively charged discharge electrodes l4 andattracted by the positively charged collecting electrodes |2. Just assoon as the particles are attracted to the inner sides of the collectingelectrodes 2, they immediately fall by means of gravity into a hopperl8, which may be periodically cleaned out by an attendant as the saidparticles collect in the hopper. Therefore, as the gas enters the bottomof the smoke stack, it is freed or cleaned of dust and other foreignparticles and thegas is emitted to the atmosphere as substantially pureand clean gas.

It is found that the best precipitating results are obtained when thedischarge electrodes 4 are of a negative polarity. This is because thespeed of the electrons is much higher than the speed of the positiveions, and. as a result, more effective ionization by the collision ofthe electrons with the suspended particles is obtained.

For the purpose of supplying uni-directional current to the dischargeelectrode l4 of the precipitating device, I employ two high-voltagerectifying tubes and 38 connected in circuit relation with the secondarytransformer winding 52 to effect a full-wave single-phase rectification.The rectifying tubes 30 and 38 are similar in construction andoperation, the former having a plate 3|, a grid 32 and a filament 33,and the latter having a plate 35, a grid 36, and a filament 31. In arectifying thermionic tube suitable for precipitating devices, thecontainer is exhausted to a high degree of vacuum and the electrodes arewell insulated and protected to withstand the high-voltage to which theyare connected. As illustrated, the filaments 33 and 31 of the rectifyingtubes 30 and 38 are electrically heated by the secondary transformerwinding 5|.

The grids 32 and 36 are connected in circuit relation with the secondarytransformer winding 53, the grid 32 being connected to the lowerterminal 60 of the secondary transformer winding 53 through means of theadjustable resistor 42 and a conductor 8|, and the grid 36 beingconnected to the upper terminal 58 of the secondary transformer winding53 through the adjustable resistor and a conductor 66. Hence, thepotential of the grids 32 and 36 may be maintained at anypredetermined'selected positive value with respect to the potential oftheir respective filaments 33 and 31. In this manner, the rectifyingtubes 30 and 38 are normally disposed to pass a predetermined amount ofcurrent for charging the discharge electrodes l4 of the precipitatingdevice H).

For the purpose of regulating the current flowing through the rectifyingtubes 30 and 38, I utilize a two-electrode thermionic tube 46, incombination with capacitors 40 and 4| and adjustable resistors 42, 43,44, and 45.

The thermionic tube 46 comprises a plate 41 and a filament 48, thelatter of which is connected by means of the conductors 12 and 13 to asecondary transformer winding 54. The mid-tap of the secondarytransformer winding 54 is connected by means of a conductor 15 to themidtap 59 of the secondary transformer winding 53. By means of thisconnection, if it were not for the opposing voltage of the secondarytransformer winding 53, the potential of the grids 32 and 36 of therectifying tubes 30 and 38 would be substantially the same as thepotential of the filament 48 of the thermionic tube 46.

The volt-ampere characteristic of the two-electrode thermionic tube 46is illustrated in Fig. 2, the plate current being plotted as ordinatesand plate voltage as abscissae. As illustrated in Fig. 2, when the platecurrent exceeds a predetermined normal value O-A, the correspondingchange in plate voltage is very rapid, because of the saturationcharacteristics of the tube. Therefore, in accordance with one of theobjects of my invention, I utilize this rapid change in plate voltage asa negative blocking bias to immediately increase the effective impedanceof the rectifying tubes 30 and 38. As a result, just as soon as 5 theplate current of the two-electrode thermionic tube 46 slightly increasesabove a predetermined normal value, the rectifying tubes 30 and 38function to reduce the value of the plate current until it again becomesstabilized at the predetermined normal value.

In explaining the operation of my control circuit, let it be assumedthat the primary winding 50 of the transformer 49 is energized from asuitable source of alternating-current. Under this 6 condition andduring the positive half of the cycle, current flows from the upperterminal of the secondary transformer winding 52 through the plate 3|and the filament 33 of the rectifying tube 38, conductor 14, the plate41 and the filament 48 of the two-electrode thermionic tube 46, theconductors 12 and 13 in parallel to the midtap of the secondarytransformer winding 54, conductors 15 and 16 to the collectingelectrodes |2 of the precipitating device l0, thence to the dischargeelectrodes l4, and a conductor 11 to the mid point 56 of the secondarytransformer winding 52. During the positive half-cycle of thealternating-current, the rectifying tube 38 is inoperative and,accordingly, passes no current.

During the negative half-cycle of the alternating-current, current flowsfrom the bottom terminal 51 of the secondary transformer winding 52through the plate 35 and filament 31 of the rectifying tube 38,conductor 14, the plate 41 and the filament 48 of the thermionic tube 46and thence, through the same circuit as just previously traced for thepositive half-cycle, to the mid-tap 56 of the secondary transformerwinding 52. So long as the smoke or the gases passing downwardly throughthe collecting electrodes I2, or as the dielectric strength between thedischarge electrodes l4 and the collecting electrodes I 2 remains at anormal value, the rectifying tubes 30 and 38 will pass a normalpredetermined value of current. Under the normal operating condition,the plate voltage of the two-electrode thermionic tube 46 is representedby the value -0 of Fig. 2, which means that if it were not for theopposing voltage of the secondary transformer winding 53, the grids 32and 36 of the rectifying tubes 30 and 38 would be negatively chargedwith respect to their filaments 33 and 31 by an amount equal to OC.However, under the normal operating conditions of the precipitatingdevice, the positive biasing voltage of the secondary trans formerwinding 53 is greater than the drop of potential across the thermionictube 46 by an amount sufficient to cause the rectifying tubes 30 and 38to pass to normal current.

However, should the density of the smoke increase to cause acorresponding decrease in the dielectric strength between the dischargeelectrodes l4 and the collecting electrodes II of the precipitatingdevice or as a result of any other condition to cause the dielectricstrength to decrease, the rectifying tubes 30 and 38 tend to pass morecurrent in order to meet the demands of the precipitating device 10.Just as soon as the current exceeds the predetermined normal value, forinstance a value O-B of Fig. 2, the plate voltage of the thermionic tube46 increases to a value OD, which may be in the order of ten to fifteentimes the normal value OC. The increase of the plate voltage of thethermionic tube 46 to a value OD opposes and greatly exceeds thepositive biasing effect of the secondary transformer winding 53, withthe" result that the grids 32 and 36 of the rectifying tubes 30 and 38are highly negatively charged with respect to their filaments 33 and 31.This means that the effective impedance of the rectifying tubes 30 and38 is materially increased, which, accordingly, opposes any increase ofthe load current above the normal predetermined operating value.Therefore, by means of the thermionic tube 46 which provides thenegative blocking bias to regulate or limit the current passing throughthe rectifying tubes 30 and 38 to a predetermined low safe value, thelives of the respective tubes are materially lengthened since the systemprevents surges and flashovers.

In order to make the regulating action of the biasing effect of thethermionic tube 46 gradual and free from too quick a change, thecombination of the capacitor 40 and the adjustable resistors 42 and 43are connected in circuit relation with the grid 32. of the rectifyingtube 30, and the combination of the capacitor 4| and the adjustableresistors 44 and 45 are connected in circuit relation with the grid 36of the thermionic tube 38. The adjustable resistors 42 and 45 predetermine the rate at which the respective capacitors become chargedwhen the load current exceeds the predetermined normal value, and theadjustable resistors 43 and 44 predetermine the rate at which therespective capacitors discharge when the load current is decreased belowthe predetermined normal value. In other words, the rapidity at whichthe rectifying tubes 30 and 38 respond to offset any change of theloadcurrent from the normal value, is somewhat dampened or delayed inorder to give a smooth and continuous regulating effect.

Since certain changes in my invention may be made without departing fromthe spirit and scope thereof, it is intended that all matters contamedin the foregoing description and shown in the accompanying drawing shallbe interpreted as illustrative and not in a limiting sense.

I claim as my invention:

1. In an electrical control system for electrical precipitators having acollecting electrode, a discharge electrode disposed in space relationwith said collecting electrode, and means for passing a gas through theregion of said electrodes, the combination of a grid-controlledrectifying tube for supplying uni-directional-current of relatively highvoltage to the discharge electrode, a source of electrical energy, meansfor connecting the rectifying tube in circuit relation with thedischarge electrode and the source of electrical energy, means fornormally biasing the grid of the rectifying tube to cause apredetermined amount of current to flow therethrough, a two-electrodeelectronic tube, and means for so interconnecting the rectifying tubeand the electronic tube that. when the current flowing through the saidtube: exceeds the said predetermined amount, the electronic tubeprovides a negative blocking bias to oppose and exceed the action of thenormal biasing means to thus decrease the current flowing through saidtubes.

2. In an electrical control system for electrical precipitators having acollecting electrode, a discharge electrode disposed in space relationwith said collecting electrode, and means for passing a gas through theregion of said electrodes, the combination of a grid-controlledrectifying tube for supplying uni-directional-current of relatively highvoltage to the discharge electrode, a source of electrical energy, meansfor connecting the rectifying tube in circuit relation with thedischarge electrode and the supply of electrical energy, means fornormally biasing the grid of the rectifying tube to cause apredetermined amount of current to flow therethrough, a two-electrodeelectronic tube, and means for so interconnecting the rectifying tubeand the electronic tube that, when the current flowing through the saidtubes exceeds the said predetermined amount, the electronic tubeprovides a negative blocking bias to oppose and exceed the action of thenormal biasing means to thus decrease the current flow- I ing throughsaid tubes, and means for predetermining the rate of decay of the saidcurrent when it exceeds the said predetermined normal value and forpredeter'mining the rate of building up when it falls below the saidpredetermined normal value.

3. In an electrical control system for electrical precipitators or thelike having a collecting electrode, a discharge electrode disposed inspace relation with said collecting electrode, and means for passing agas through the region of said electrodes, the combination of arectifying tube having a plate, a filament, and a grid for supplying auni-directional-current of relatively high voltage to the dischargeelectrode, a two-electrode electronic tube associated with therectifying tube, a transformer having a primary winding and a pluralityof secondary windings, means for connecting the plate and the filamentof the rectifying tube and the two-electrodes of the electronic tube incircuit relation with the discharge and the collecting electrodes andone of the secondary windings of the transformer, means includinganother secondary winding of the transformer for normally biasing thegrid of the rectifying tube relative to the filament to cause apredetermined amount of current to flow through said tubes, means for sointerconnecting the two electrodes of the electronic tube in circuitrelation with the filament and the grid of the rectifying tube that,when the current flowing through the said tubes exceeds the saidpredetermined value, the increase in potential between the twoelectrodesof the electronic tube causes a negative blocking bias to oppose andexceed the action of the normal biasing means to thus decrease thecurrent flowing through the said tubes.

4. In an electrical control system for electrical precipitators having acollecting electrode, a discharge electrode disposed in space relationwith said collecting electrode, and means for passing a gas through theregion of said electrodes, the combination of a rectifying tube having aplate, a filament, and a grid for supplying uni-directional-current ofrelatively high voltage to the discharge electrode, a two-electrodeelectronic tube associated with the rectifying tube, a transformerhaving a primary winding and a plurality of secondary windings, meansfor connecting the plate and the filament of the rectifying tube and thetwo-electrodes of the electronic tube in circuit relation with thedischarge and" the collecting electrodes and one of the secondarywindings of the transformer, means including another secondary windingof the transformer for normally biasing the grid of the rectifying tuberelative to the filament to cause a predetermined amount of current toflow through said tube, means for so interconnecting the two-electrodesof the electronic tube in circuit relation with the filament and thegrid of the rectifying tube that, when the current flowing through thesaid tubes exceeds the said predetermined value, the increase inpotential between the two-electrodes of the electronic tube causes anegative blocking bias to oppose and exceed the action of the normalbiasing means to thus decrease the current flowing through the saidtubes, a capacitor connected between the filament and the grid of therectifying tube, a resistor. connected in series circuit relation withsaid capacitor to predetermine the rate at which the said capacitorbuilds up, and a second resistor connected in parallel circuit 7relation with said capacitor to predetermine the sive to theuni-directional current delivered to the load, the characteristics ofthe electronic tube being such that, when the uni-directional currentexceeds a predetermined value, the increase in voltage across the saidtube causes a nega-- tive blocking bias to regulate the rectifying tube.

6. A control system for supplying a uni-directional current ofrelatively high potential to a load comprising, in combination, a sourceof alternating current, a load circuit that is disposed to operate on auni-directional current, a gridcontrolled rectifying tube connectedbetween said source and said load to convert alternating-current touni-directional-current, means for normally biasing the grid of therectifying tube to causethesaid tube to pass a predetermined amount ofuni-directional current, an electronic tube associated with therectifying tube and responsive to the uni-directional-current deliveredto the load, the characteristics of the electronic tube being such that,when the uni-directional-current exceeds the said predetermined value,the increase in voltage across the said tube causes a negative blockingbias to oppose and exceed the action of the normal biasing means toregulate the rectifying tube.

7. A control system for supplying a uni-directional current ofrelatively high potential to a load' comprising, in combination, asource of alternating current, a load circuit that is disposed tooperate on a uni-directional current, a gridcontrolled rectifying tubeconnected between said source and said load to convertalternatingcurrent to uni-directional-current, means for normallybiasing the grid of the rectifying tube to cause the said tube to pass apredetermined amount of uni-directional current, an electronic tubeassociated with the rectifying tube and responsive to theuni-directional-current delivered to the load, the characteristics ofthe electronic tube being such that, when the uni-directionalcurrentdelivered to the load exceeds the said predetermined value, the increasein voltage across the said electronic tube causes a negative blockingbias to oppose and exceed the action of the normal biasing means toregulate the rectifying tube, and means including a capacitor andresistor to cause the regulating action of the rectifying tube to be ofany predetermined rate.

8. A control system for supplying a uni-directional current ofrelatively high potential to a load comprising, in combination, a sourceof alternating current, a load circuit that is disposed to operate on auni-directional current, a plurality of grid-controlled rectifying tubesconnected between said source and said load to convert alternatingcurrent to uni-directional-current, means for normally biasing the gridof the rectifying tubes to cause the said tubes to pass a pre-determinedamount of uni-directional current, an electronic tube associated withthe rectifying tubes and responsive to the uni-directional currentdelivered to the load, the characteristics of the thermionic tube beingsuch that, when the uni-directional current exceeds a pre-determinedvalue, the increase in voltage across the said tube causes a negativeblocking bias to regulate the rectifying tubes, and means including acapacitor and resistors to cause the regulating action of the rectifyingtubes to be of any predetermined rate.

9. In an electrical control system for electrical precipitators having acollecting electrode, a discharge electrode disposed in space relationwith said collecting electrode, and means for passing a gas through theregion of said electrodes. the

combination of a plurality of rectifying tubes each having a plate, afilament, and a grid for supplying a uni-directional current ofrelatively high voltage to the discharge electrode, a twoelectrodeelectronic tube associated with the rectifying tubes, a transformerhaving a primary winding and a plurality of secondary windings, meansfor connecting the plate and the filament of the rectifying tubes andthe two-electrodes of the electronic tube in circuit relation with thedischarge and the collecting electrodes and one of the secondarywindings of the transformer, means including another secondary windingof the transformer for normally biasing the grids of the rectifyingtubes relative to their filaments to cause a predetermined amount ofcurrent to flow through said tubes, means for so interconnecting thetwo-electrodes of the electronic tubes in circuit relation with thefilaments and the grids of the rectifying tubes that, when the currentflowing through the said tubes exceeds the said predetermined value, theincrease in potential between the two-electrodes of the electronic tubecauses a negative blocking bias to oppose and exceed the action of thenormal biasing means to thus decrease the current flowing through thesaid tubes.

10. In an electrical control system for electrical precipitators havinga collecting electrode, a discharge electrode disposed in space relationwith said collecting electrode, and means for passing a gas through theregion of said electrodes, the combination of a rectifying tube having aplate, a filament and a grid for, supplying a uni-directional-current ofrelatively high voltage to the discharge electrode, a source ofelectrical energy, means for connecting the plate and the filament incircuit relation with the discharge electrode and the source ofelectrical energy, and a twoelectrode electronic tube connected inseries circuit relation with the filament and plate of the rectifyingtube to regulate the bias of the grid of the rectifying tube, thecharacteristic of the electronic tube being such that a small change inthe current flowing between the twoelectrodes causes a relatively largechange in the grid bias.

DEWEY D. KNOWLES.

